How can temperature affect reaction rate?

1 Answer
Dec 28, 2013

Particles can react only when they collide. If you heat a substance, the particles move faster and so collide more frequently. That speeds up the rate of reaction.

Collisions result in a reaction only if the particles collide with enough energy to get the reaction started. The critical amount of energy to make the reaction proceed is called the activation energy, Ea.

The diagram above shows the potential energy changes as unstable methyl isocyanide rearranges to form the more stable methyl cyanide. The energy barrier keeps the unstable CH₃NC from reacting quickly, even though energy is released during the reaction.

The activation comes from the kinetic energy of the molecules. At a given temperature, the molecules have a distribution of energies with a well-defined average. This energy distribution curves for two different temperatures look like this:

Only those particles represented by the area to the right of the activation energy will react when they collide. The great majority don't have enough energy to react. They will simply bounce apart. To speed up the reaction, you need to increase the number of the highly energetic particles — those with energies equal to or greater than the activation energy. Increasing the temperature has exactly that effect.

The curve labelled T₁ is at some original temperature. The curce labelled T₂ is at a higher temperature. If you now mark the position of the activation energy, you can see that, although the curves haven’t changed much, there has been such a large increase in the number of the very energetic particles that many more now collide with enough energy to react. The area under the T₂ curve to the right of the activation energy appears to have at least doubled. Therefore, the rate of the reaction has also doubled.

A rough rule of thumb is that a 10 °C increase in temperature approximately doubles the rate of reaction.